Ever increasing urban atmospheric pollution, due to exhaust gases of automobiles, is one of the public hazards which is drawing near to the limit of the maintenance of health of city dwellers. In an attempt to solve this problem, catalysts have been utilized in mufflers for cleaning and purifying exhaust gases of cars. However, catalytic products capable of exhibiting satisfactory performance have not yet been produced because the following three requirements have not been met:
The first of these requirements is that the catalyst should be thermally stable and not lose its activity. This thermal stability has two meanings. One of them is that the start-up temperature leading to the exhibition of the catalyst activity should be low. It is sufficient if the catalyst does not act effectively unless the engine is warmed and the exhaust system is sufficiently heated. It is necessary that the catalyst should have thermal stability at high temperatures above 870.degree. C. when the temperature rises or great quantities of imperfectly burned products are formed. Hence, the catalyst should first meet the stability requirement in these two senses.
A second requirement is that the catalyst should be chemically stable. This means that the catalyst should not lose its activity by covering of its surface either chemically or physically with lead halides or oxyhalides of various vapor pressures which are produced from tetraalkyl leads, most commonly used anti-knocking agents for gasoline, and halogen compounds used for preventing the accumulation, in the combustion chamber of the engine, of oxides formed by the combustion of these anti-knocking lead compounds.
A third requirement is that the catalyst should be physically durable as well as chemically stable. This requirement is associated with a catalyst carrier rather than with the catalyst per se. The loss of a catalyst, as generally understood, results from the fact that the catalyst powder physically adsorbed on, or chemically bound to, the surface of the carrier, is scattered in the form of powder owing to the following reasons: friction is caused between the carriers or between the carriers and the catalyst structure; the carrier itself is destroyed by the stress caused by repeated thermal load; and the carrier is broken by the internal pressure which occurs when the water adsorbed to the porous carrier is swollen by an abrupt thermal load.
Especially in the case of purifying exhaust gases from cars, the aforesaid requirements, particularly the second and third, become very strict conditions. Thus, an internal combustion engine produces exhaust gas having a high temperature, high current speed and high pulsation, especially at the time of high load, such as at the time of acceleration, or ascending an upward slope. In such a case, gas having a current speed of as high as 70m a second is exhausted at a temperature exceeding 1000.degree. C. Then, the pulsation inside the exhaust pipe becomes as high as above 200 times a second due to a pressure difference of above 0.7 kg/cm.sup.2. In addition, mechanical vibration acts on the exhaust pipe. It is required that a car exhaust gas treating apparatus should carry out the expected cleaning and purifying action under such thermally and physically strict conditions of use. Moreover, the cleaning action should continue in full effect throughout the car driving distance of tens of thousands of kilometers, and this action should not offer resistance to discharge of exhaust gas.
These conditions of use pose problems which are difficult to solve, in that an internal combustion engine, provided with a catalytic converter, must operate properly under driving conditions which change abruptly and frequently, as in an automobile running in a city and subjected to stop, start and acceleration at frequent intervals. In such a case, the catalytic converter, whose object is to treat the exhaust gas, is subjected to special conditions which it must endure, including conditions which change irregularly and drastically. Nevertheless, the converter must effectively carry out predetermined cleaning and purifying of exhaust gases generated over a long period of time.
With a view to meeting these conditions, catalytic converters using a pellet-type catalyst and catalytic converters using a flame retardant fibred catalyst carrier, such as asbestos or kaolin, have been proposed. However, none of these proposals has satisfied the aforesaid three requirements and conditions. Especially for treating car exhaust gas, these converters have been completely unsatisfactory.
To obviate these disadvantages, a number of experiments have been made to increase the caking property or hardness of the carrier or to include the catalyst in the interior of the carrier by mixing the catalyst powder, the carrier powder and a caking agent together, with a view to preventing loss of catalyst due to the surface friction of the carrier in a pellet-type catalyst. These experimental studies, however, have not led to the production of catalyst having sufficient physical durability.
Hauel, U.S. Pat. No. 3,189,563 discloses a catalytic converter utilizing glass fibers as a catalyst carrier and, in this respect, bears some resemblance to the present invention. However, in accordance with the Hauel disclosure, the surface of a commercial glass fiber is coated with ceramics or refractory powder before formation of the carrier. In accordance with the Hauel disclosure, the ceramics are initially precoated as a surface treating agent, because it is difficult to directly coat a catalyst onto the surface of glass fiber, irrespective of whether or not an adhesive or the like is used. For this reason, in the Hauel arrangement, ceramics are pre-coated in advance on the surface of glass fiber, and the catalyst is coated on the surface of the ceramics. With the Hauel method, the degree of heat resistance of commercial glass fiber, which has a relatively low melting point, is increased.
In the method of coating ceramics or refractory in advance on the surface of glass fiber, as taught in Hauel, it is stated that a catalyst film or powder prevents attrition due to pulsation in exhaust gas of an automobile. However, because the flexibility of glass fiber is lost by these ceramic coatings, and the glass fiber is solidified, there is a danger that the glass fiber may be broken by mechanical (physical) shock, and it is not possible to continue stable operations for a long period of time. This constitutes a distinct disadvantage of the Hauel catalytic converter.